1. Field of the Invention
This invention relates to seismic exploration in offshore areas and more particularly to a system for testing seismic streamers used in offshore exploration for oil and gas structures.
2. Description of the Prior Art
Many of the most promising areas for exploration for oil or gas are in offshore areas. Seismic surveys conducted from vessels from which streamers of hydrophone channels are towed are an important tool used in locating and evaluating offshore structures which may contain oil or gas. Signals picked up by the hydrophones are transmitted through the streamer back to the exploration vessel where they are recorded for subsequent interpretation.
The streamers towed behind the exploration vessels may include, for example, 48 to 96 hydrophone channels for sections connected end to end. Each channel is usually 50 to 250 feet long; consequently, the streamer may be two miles or more in length. Typically, 32 hydrophones are connected in parallel in each channel. The channels include a tube of rubber, vinyl chloride or other elastic material housing the hydrophones. The tubes are filled with a liquid of high electrical resistivity such as highly refined kerosene to transmit impulses received in the seismic operations to the hydrophones and to mimimize invasion of the channel by sea water. A matching transformer in each channel transforms the signal developed by the primary circuit in the channel for transmission from the channel.
The channels are connected together end to end at boots to form the streamer. Electrical connections are carried from each matching transformer through each of the channels and boots nearer the vessel to the vessel for separate recording of the signals from each of the channels. Suitable connections of the electrical conductors are made in each of the boots. Strain cables for carrying the mechanical load of the streamer caused by drag as the streamer is towed extend from the vessels through each of the channels and boots.
During use of the streamer, salt water may invade a channel and reduce the current leakage resistance of the primary circuit in the channel. The lower resistance in the channel causes a loss of sensitivity as well as a loss of low frequency response and a phase shift. Because the signals of most value in seismic exploration are in the low 8 to 60 hertz range, means for checking the channels to determine whether or not there has been salt water invasion are particularly desirable. Moreover, during the assembly of a streamer, either initially or after repairs, there may be reversal of polarity of the signal delivered to the vessel because of reversals in connection of the cables. Such reversals of polarity can add greatly to the difficulty in interpretation of the records made during the seismic survey.
In U.S. Pat. No. 2,918,651 of Podolak et al and U.S. Pat. No. 3,659,255 of Trott apparatus is described for calibrating hydrophones for use in offshore exploration. In the apparatus described in each of the patents, a test is made on a single hydrophone. No indication is given of the output of the primary circuit of a channel of a streamer when a plurality of hydrophones are assembled in a channel. In U.S. Pat. No. 3,864,664 of Trott et al, apparatus is described which will allow testing individual hydrophones as assembled in seismic streamers. The apparatus consists of a tubular element that may be mounted on shipboard and through which the streamer is pulled. Each hydrophone element in the arrangement that is to be checked is positioned centrally in the testing apparatus. It is apparent that testing an entire seismic streamer will be a time-consuming process as the two miles or more of oil-filled tubes must be pulled through the calibrator.